Diphtheria toxin (DT) is secreted by toxigenic strains of the corynebacterium diphtheriae as a single polypeptide chain of 58 dKA and contains three structurally and functionally distinct domains: the receptor binding domian (R, residues 387–535), the pore-forming/membrane translocating domain (T, residues 200–378) and the catalytic domain (C, residues—188). After binding its cellular receptor, HBEGF, a proteolytic activation event cleaves DT into two fragments which remain tightly associated through a single disulfide bond and noncovalent interactions. Exposure of DT to the acidic environment of endosomes triggers a conformational change which drives the insertion of the T domain into the lipid bilayer, forming a pore through which the C domain is translocated into the cytoplasm. Once free in the cytoplasm, diphtheria toxin's C domain inhibits protein synthesis by specifically ADP-ribosylating elongation factor 2. While many of the toxins within this class contain all three functional domain within a single synthesized polypeptide chain, some toxin systems, such as anthrax, have separated these functions into two or three cooperating proteins. Mechanistically, the initial insertion of the T domain resembles the early events of both the fusion and lysogenic polypeptides, whereby environmental ques trigger the exposure of hydrophobic domains capable of membrane insertion. However, the second phase of DT translocation more closely resembles cellular protein transport systems which utilize proteinaceous, aqueous channels as conduits through which partially unfolded, hydrophilic proteins can be translocated. For example, cotranslational movement of proteins into the endoplasmic reticulum utilize a dedicated channel formed by the Sec61 protein complex and similar channel systems have been described for both mitochondrial and peroxisomal protein import. Like its cellular counterpart, DT's T domain forms a pore of limited size and requires at least the partial unfolding of translocating proteins.
Thus, cells ectopically expressing HBEGF are capable of translocating toxin into the cytoplasm and acute cytotoxicitiy quickly ensues as the C domain proceeds to inhibit cellular protein synthesis by inactivating elongation factor 2 (EF-2). Cells lacking HBEGF on their surface are spared this fate and continue to thrive even in the presence of relatively high concentrations of free DT.
Immunoglobulins must bind to a vast array of foreign molecules and thus exist in many forms. The sequence of the variable (V) region of immunoglobulin molecules varies tremendously, conferring virtually unlimited capacity to bind antigens. The constant (C) region comes in five different varieties: α, δ, ε, γ and μ, providing five different isotypes: IgA, IgD, IgE, IgG and IgM, each of which performs a different set of functions. B cells initially produce only IgM and IgD, and must be activated or induced to produce the other isoforms, such as IgE.
The course of IgE production starts with the activation of B cells. Upon activation with an antigen, B cells follow one of two differentiation pathways: they may differentiate directly into plasma cells, which are basically antibody-secreting factories, or they may give rise to germinal centers, specialized structures within lymphoid organs. In the latter, successive rounds of mutation of the V region genes is followed by expression of the gene products on the cell surface, with selection of the cells on the basis of the affinity of the mutated immunoglobulins against the antigen.
In both pathways of antigen-induced B cell differentiation, isotype switching occurs in which the C region of the immunoglobulin heavy chain changes from the joint expression of IgM and IgD on naive B cells to expression of one of the downstream isotypes such as IgE. This switching involves the replacement of upstream C regions with a downstream C region that has biologically distinct effector functions without changing the structure of the variable portion and, hence, its specificity. For IgE switching, a deletional rearrangement of the Ig heavy chain gene locus occurs, a rearrangement that joins the switch region of the μ gene, Sμ, with the corresponding region of the ε gene, Sε. This switching is minimally induced by IL-4 or IL-13, which initates transcription through the Sε region, resulting in the synthesis of germ-line (or “sterile”) ε transcripts; that is, transcripts of the unrearranged Cε heavy genes. This IL-4 induced transcription is inhibited by IFN-γ, IFN-α, and TGF-β. A second signal, normally delivered by T cells, is required for actual switch recombination leading to IgE production. The T cell signal may be replaced by monoclonal antibodies to CD40, Epstein-Barr viral infection, or hydrocortisone.
Recently, the mechanism of class switch recombination has been explained by an accessibility model, wherein the specificity of the switch gene rearrangement is determined by the modulation of switch region accessibility; that is, the opening up of the chromatin in certain areas, allowing the required protein/enzyme complexes access to the genes.
IgE antibodies are crucial immune mediators of allergic reactions, and have been shown to be responsible for the induction and maintenance of allergic symptoms. For example, the introduction of anti-IgE antibodies has been shown to interfere with IgE function, thus working to alleviate allergic symptoms. See Jardieu, Current Op. Immunol. 7:779–782 (1995), Shields et al., Int. Arch. Allergy. Immunol. 107:308–312 (1995).
Accordingly, it is an object of the invention to provide compositions and methods useful in screening for modulators of IgE production, in particular for modulators of switch rearrangement.